Loop optimisation with identical counters

[Simon Peyton-Jones]

Interesting.  What would it look like in Core?  Anyone care to make a ticket?

S

|  -----Original Message-----
|  From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-
|  bounces at haskell.org] On Behalf Of Roman Leshchinskiy
|  Sent: 03 November 2010 10:55
|  To: Christian Hoener zu Siederdissen
|  Cc: glasgow-haskell-users at haskell.org
|  Subject: Re: Loop optimisation with identical counters
|  
|  LLVM doesn't eliminate the counters. FWIW, fixing this would improve performance of
|  stream fusion code quite a bit. It's very easy to do in Core.
|  
|  Roman
|  
|  On 3 Nov 2010, at 10:45, Christian Hoener zu Siederdissen
|  <choener at tbi.univie.ac.at> wrote:
|  
|  > Thanks, I'll do some measurements on this with ghc7.
|  >
|  > Gruss,
|  > Christian
|  >
|  > On 11/02/2010 01:23 PM, Simon Marlow wrote:
|  >> On 02/11/2010 08:17, Christian Höner zu Siederdissen wrote:
|  >>> Hi,
|  >>>
|  >>> is the following problem a job for ghc or the code generation backend
|  >>> (llvm)?
|  >>>
|  >>> We are given this program:
|  >>>
|  >>> {-# LANGUAGE BangPatterns #-}
|  >>>
|  >>> module Main where
|  >>>
|  >>> f :: Int ->  Int ->  Int ->  Int ->  Int
|  >>> f !i !j !s !m
|  >>>   | i == 0    = s+m
|  >>>   | otherwise = f (i-1) (j-1) (s + i+1) (m + j*5)
|  >>>
|  >>> g :: Int ->  Int
|  >>> g !k = f k k 0 0
|  >>>
|  >>>
|  >>> ff :: Int ->  Int ->  Int ->  Int
|  >>> ff !i !s !m
|  >>>   | i == 0    = s+m
|  >>>   | otherwise = ff (i-1) (s + i+1) (m + i*5)
|  >>>
|  >>> gg :: Int ->  Int
|  >>> gg !k = ff k 0 0
|  >>>
|  >>> main = do
|  >>>   print $ g 20
|  >>>   print $ gg 20
|  >>>
|  >>>
|  >>> Here, 'f' and 'g' are a representation of the code I have. Both counters
|  >>> 'i' and 'j' in 'f' count from the same value with the same step size and
|  >>> terminate at the same time but are not reduced to just one counter. Can
|  >>> I reasonably expect this to be done by the code generator?
|  >>> 'ff' represents what I would like to see.
|  >>
|  >> GHC doesn't have any optimisations that would do this currently,
|  >> although it's possible that LLVM's loop optimisations might do this on
|  >> the generated code for f.
|  >>
|  >> Cheers,
|  >>    Simon
|  >>
|  >>
|  >>
|  >>> Btw. look at the core, to see that indeed 'f' keep four arguments.
|  >>> Functions like 'f' are a result of vector-fusion at work but can be
|  >>> written by oneself as well. The point is that if 'f' gets reduced to
|  >>> 'ff' then I can have this:
|  >>>
|  >>> fun k = zipWith (+) (map f1 $ mkIdxs k) (map f2 $ mkIdxs k)
|  >>>
|  >>> which makes for nicer code sometimes; but before rewriting I wanted to
|  >>> ask if that kills performance.
|  >>>
|  >>>
|  >>> Thanks,
|  >>> Christian
|  >>>
|  >>>
|  >>>
|  >>> _______________________________________________
|  >>> Glasgow-haskell-users mailing list
|  >>> Glasgow-haskell-users at haskell.org
|  >>> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
|  >>
|  >
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